Manganese in many forms is used in a variety of industrial and other applications. It is the fourth most used metal in terms of tonnage, after iron, aluminum and copper. As a trace element, it is found in the body, and also has a variety of uses in industry. For example, manganese dioxide is used in dry cell batteries, in aluminum cans, and in the electronic components of television sets.
Manganese is most often extracted from seawater and other natural sources, but can also be separated from other metals found in aqueous solutions that are byproducts of many industrial processes. For example, geothermal steam and hot brines are found in naturally occurring, large subterranean reservoirs throughout the world. In many areas where extraction is convenient, the steam and hot brines provide a partially renewable resource for the production of power. The pressurized, hot geothermal brines are extracted from the earth to generate power by using steam flashed off from the brine to power a turbine. Thereafter, metals such as manganese can be extracted from the brine before it is returned to the ground.
One of the general problems encountered with the extraction of metals from aqueous solutions involves changes in pH associated with the exchange of metal ions for hydrogen ions in ion exchange reactions. This causes a progressive lowering of the pH which in turn impedes the efficiency of the process. Attempts to solve this problem have been reported, but their success has been limited. See, for example, U.S. Pat. No. 4,128,493, which reports the use of organic solvents and quaternary ammonium salts to extract metals from acidic solutions.
Other methods for recovering metals such as zinc from geothermal brine include precipitation with sulfides and various combinations of solvent extractions and electromagnetic stripping. However, continuous batch processes using these methods are limited due to scaling of the equipment due to the presence of large amounts of silica.
Manganese is usually extracted in the form of electrolytic manganese dioxide (EMD). Perhaps the most common process being used today for extracting manganese is by mixing the manganese-containing material with sulfuric acid to form a manganese sulfate electrolyte. This intermediate is separated from other metals by precipitation and filtration. Thereafter, the manganese sulfate is subjected to solvent extraction and electrowinning. See, for example, PCT WO 99/14403; and L. A. Mel'nik, et al., Russian J. of Electrochemistry 32: 248-51 (1996).
The current sulfate process for the production of electrolytic manganese dioxide (EMD) was invented more than seventy years ago in the US. However, it was commercialized in the early 1950's in a move largely driven by the US military seeking higher quality batteries for use in Korea. The standard sulfate process is still the only commercial process for the manufacture of EMD.
The only significant market for EMD is its use in dry cell batteries (small amounts are also consumed in the production of soft ferrites for the electronics industries). Optimum battery performance is based on a combination of chemical, physical and electrical characteristics. However, the key feature of EMD which sets it apart from other manganese dioxides is its crystal structure. This all important characteristic is developed during high temperature aqueous electrolysis, the heart of the EMD manufacturing process. Although a typical specification for EMD might have about 20 components, EMD quality is defined by 4 key criteria: Crystal structure (disordered, hydrated, non-stoichiometric); Chemical purity (minimum 92% MnO2 (remainder essentially water) with key impurities at the single digit ppm level); Density (higher the better, since batteries are fixed volume devices); and Intrinsic discharge capacity (measured in mAh/g MnO2, again higher the better).
The standard sulfate process for EMD has been undergoing continual development since it was first commercialized in the 1950s. See e.g., Nathsarma, et al., Hydrometallurgy 45: 169-79 (1997); Alexperov et al., Journal of Applied Chemistry of the USSR 65: 2342-44 (1992). These developments have been largely driven by demands for improvements in product quality. However, the key elements of the process are unchanged.
One of the largest challenges facing the EMD industry is waste management. The best manganese ores available to the industry contain only 50% Mn. Insoluble gangue from the ore combined with wet filter cakes from process purification and filtration stages typically generate some 2 to 3 tons of solid waste per ton of EMD product. While manganese occurs widely in nature and is not generally considered a toxic element, solid wastes containing soluble manganese must be immobilized and contained in sealed dump sites to prevent ground water contamination. A large proportion of the world's EMD capacity is located in environmentally sensitive regions, such as Japan, Europe, USA and Australia. Containment of waste is a major limitation to expansion for many existing producers.
A second limitation of the standard sulfate process is the low current density, typically about 55 A/m2, at which plants must operate, although some plants operate between about 50 A/m2 to 70 A/m2. This is one fifth to one tenth the current density normally associated with metal electrowinning processes. It is known that a chloride electrolyte supports higher current densities for the production of EMD, in the order of 80 to 100 A/m2, improving plant productivity and reducing capital cost per annual product ton. However, a chloride electrolyte system has not yet been adopted.
Accordingly, there is a need for a more efficient process for extracting EMD from various natural and industrial sources that is more compatible with environmental concerns and commercial needs. The present process, based on the recovery of manganese units from liquid brine and electrolysis of a chloride liquor, has the potential to overcome or minimize these two limitations of the standard sulfate process.